1. Field of the Invention
The present invention relates generally to a method and apparatus for providing channel state information.
2. Description of the Related Art
FIG. 1 is a block diagram of a prior art digital video broadcasting terrestrial (DVB-T) transmitter. The DVB-T transmitter of FIG. 1 processes a Moving Picture Experts Group (MPEG) bit stream 1 of a DVB-T signal and transmits a resulting signal into the air via an antenna. The DVB-T transmitter includes an energy dispersal unit 2, an outer coder 3, an outer interleaver 4, an inner coder 5, an inner interleaver 6, a signal mapper 7, a frame adaptation unit 8, an OFDM modulator 9, a digital-to-analog converter (DAC) 10 and a transmitter front-end 11. The signal mapper 7 generates in-phase (I) and quadrature (Q) signals according to a modulation format such as quadrature phase shift keying (QPSK), 16-quadrature amplitude modulation (QAM), 64-QAM, etc. The frame adaptation unit 8 changes these signals to a frame structure. Each frame consists of 68 OFDM symbols. Each OFDM symbol consists of 6817 active carriers (in 8 k mode) or 1705 active carriers (in 2 k mode). This frame structure is a standard of European Telecommunication Standard Institute (ETSI).
FIG. 2 illustrates a scattered pilot insertion pattern in a prior art DVB-T system. In addition to changing the modulated signals to a frame structure, the frame adaptation unit 8 inserts a Continual Pilot Carrier (CPC), a Scattered Pilot Carrier (SPC) and Transmission Parameter Signaling Carriers (TPSCs) into the frame structure. Positions of these carriers are predetermined, and, as shown in FIG. 2, a scattered pilot insertion pattern has a form in which every fourth symbol is the same (see SYMBOL #00 and SYMBOL #04, for example).
FIG. 3 is a block diagram of a prior art DVB-T receiver. The DVB-T receiver of FIG. 3 processes an aerial wave received via an antenna 13 by a procedure inverse to that of the transmitter shown in FIG. 1, and transmits a generated MPEG bit stream 27 to a downstream MPEG bit stream processor. The DVB-T receiver includes a tuner 14, an ADC 15, an OFDM demodulator 16, a synchronization unit 17, a channel equalizer 18, a television par satellite (TPS) decoder 19, a bit metric calculator and inner de-interleaver 22, a channel state information (CSI) processor 24, a Viterbi decoder 25 and an outer de-interleaver, decoder and de-randomizer 26.
The channel equalizer 18 outputs an equalized complex OFDM signal and a squared magnitude of a channel frequency response (hereinafter, referred to as an “SMCFR”). The CSI processor 24 estimates the degree of certainty for each of the carriers of an OFDM signal and outputs a CSI value. The CSI value is generally a signal-to-noise ratio of a sub-carrier.
FIG. 4 is a block diagram of the bit metric calculator and inner de-interleaver 22 of FIG. 3. Here, the 64-QAM transmission mode is used. The bit metric calculator and inner de-interleaver 22 outputs symbols obtained by processing the CSI value and the I and Q signals received from the channel equalizer 18 to the Viterbi decoder 25. The bit metric calculator and inner de-interleaver 22 includes a symbol de-interleaver 28, bit metric calculators 29-34, bit de-interleavers 35-40 and a bit multiplexer 41.
FIG. 5 is a view for explaining bit metric calculation. A bit metric is calculated through de-mapping shown in FIG. 5, based on the following expression (1):BMi=CSIk×(|Rk−S0|2−|Rk−S1|2)  (1).In Expression (1), BMi is an ith bit metric, Rk is a complex value of a kth carrier, and S0 is a value corresponding to ‘0’ at an ith position, as a complex value of a nearest point on an I-Q constellation plot (constellation plot of In-phase and Quadrature components). S1 is a value corresponding to ‘1’ at the ith position, as a complex value of a nearest point on the I-Q constellation plot, and CSIk is a CSI signal of the kth carrier. FIG. 5 shows an example of a 16-QAM transmission mode.
FIG. 6 is a graph of bit error rate in a channel with co-channel interference versus SIR; and FIG. 7 illustrates spectrums for frequency responses of a DVB-T signal and an analog broadcasting signal. CSI measurement methods include an indirect measurement method, a direct measurement method and a combined direct-indirect method. The DVB-T receiver of FIG. 3 adopts an indirect method for channel state measurement using the SMCFR calculated in channel equalizer 18.
Referring to FIG. 6, the indirect measurement method exhibits good performance in a channel with white noise or in a static channel. The indirect method does not exhibit good performance in a channel with frequency selective interference. For example, co-channel interference in a co-channel may be a case where an analog TV signal is mixed into the DVB-T signal and which has a spectrum as shown in FIG. 7. In such a case, the indirect method of channel state information measurement exhibits poor performance due to the presence of co-channel interference. This is because the indirect method cannot provide sufficiently accurate channel state information to achieve a desired SNR with a desired low bit error rate (BER) after Viterbi decoding, in the presence of the co-channel interference.
FIG. 8 is a graph of bit error rate in a multi-path channel versus SNR. FIG. 8 illustrates the problem with direct measurement method performance in a channel with white noise, such as a multi-path channel. Referring to FIG. 8, the indirect method of channel state measurement obtains an SNR gain of about 3.3 dB at a bit error rate of 2*10−4, as compared to a direct method of channel state measurement.
In order to have good performance in a channel with frequency selective interference (co-channel interference), there have been attempts at the direct method of channel state measurement and a combined direct-indirect method of channel state measurement. The direct method of channel state measurement uses differences between a received signal value and the nearest points in an I-Q constellation plot, as shown in FIG. 5. A direct measurement method is disclosed in detail in U.S. Pat. No. 5,636,253 and European Patent No. EP 0991239. The direct method exhibits good performance in a channel with frequency selective interference (co-channel interference), but does not exhibit good performance in a channel with white noise or in a static channel.
A combined method of channel state measurement is suggested in European Patent No. EP 1221793. However, the method disclosed in the EP '793 patent does not appear to exhibit improved performance, for a channel with frequency selective interference, as compared to performance using the indirect method.